My rack mounted Power 6 M25's have the flexible arm attachment on it that
the cables wrap through before plugging into the machine. The last step
is to go through a blue plastic clamp that is on the slide that the unit
is on itself.
Does your slides have these or was some alternative slide used?
The SAS drive frame has a U shaped piece of flat metal that snugly holds
in the power cable. If you look at the figure on (printed) page 24 for E1
and E2 you'll see it here
Again, that's printed page 24 or pdf page 32.